1. Field of the Invention
The present invention relates to a cellular mobile telephone terminal, and more particularly to the configuration of an attenuator for use in a radio frequency section in a radio transceiver section of a cellular mobile telephone terminal.
2. Description of the Related Art
In CDMA systems (for example, IS-95), signal power arriving at a base station from any cellular mobile telephone terminal must be controlled to a constant value regardless of the distance between the cellular mobile telephone terminal and the base station. To achieve this, gain control is performed in the transmitter section of each cellular mobile telephone terminal.
FIG. 13 schematically shows the relationship of the locations of cellular mobile telephone terminals relative to a base station. In FIG. 13, the cell range CL of one base station BS is several tens of kilometers in radius, for example, about 30 km in radius. Within the cell range CL of the base station BS are located many cellular mobile telephone terminals TH1, TH2 operating in different communication conditions, for example, at different distances from the base station BS or under different geographical or terrestrial conditions. These many cellular mobile telephone terminals TH1, TH2 are simultaneously performing communications with the base station BS while moving toward or away from the base station BS and under communication conditions changing from moment to moment.
In this case, if control is to be performed so that the signal power arriving at the base station BS from any cellular mobile telephone terminal will be the same whether the terminal is located farthest away from the base station or nearest to it, then considering the size of the cell range CL, the transmitter section of each cellular mobile telephone terminal is required to have a gain control width of 70 dB or greater and a high linearity of xc2x11 dB. This situation is known as the near-far problem.
If the gain control at the transmitter section of the cellular mobile telephone terminal is not performed properly, the signal power arriving at the base station becomes greater as the distance between the cellular mobile telephone terminal and the base station decreases; as a result, leakage power to adjacent channels increases, increasing the bit error rate and degrading the communication quality. In FIG. 14, solid lines A1 to A6 indicate the received signal power levels at the base station from various channels, and dashed line B4 shows the intermodulation distortion characteristic of the channel A4. FIG. 14 shows the case where the received signal power levels from the channels A3 and A5 are masked by the distortion component of the channel A4 indicated by the dashed line B4, so that correct data cannot be recovered from the channels A3 and A5 adjacent to the channel A4.
To maintain a high carrier-to-noise ratio (C/N), it is desirable that the gain control in the transmitter section of each cellular mobile telephone terminal be performed, as much as possible, in the radio frequency range where the carrier signal level is high. The reason is that, at radio frequencies, the carrier signal level is far higher than the background noise level and, if the gain is lowered in the radio frequency section, a high carrier-to-noise ratio can be maintained. On the other hand, at intermediate frequencies, the carrier signal level is low and, if the gain is lowered in the intermediate frequency section, the difference between the carrier signal level and the ground noise level becomes very small, and this difference between the carrier signal level and the noise level in the intermediate frequency section is carried over directly into the radio frequency section.
However, there has not been available an attenuator, for use in a radio frequency section, that can by itself accomplish gain control with a linearity of xc2x11 dB over a wide range of 70 dB or greater. In the prior art, to accomplish gain control with a linearity of xc2x11 dB over a wide range of 70 dB or greater in the radio transmitter section of a cellular mobile telephone terminal, it has been practiced to control the gain in step-like manner in the radio frequency section while continuously controlling the gain in the intermediate frequency section. When the amount of gain control accomplished in the radio frequency section and that accomplished in the intermediate frequency section are used in combination as described above, gain control with a linearity of xc2x11 dB can be achieved over a wide range of 70 dB or greater.
The gain control at the cellular mobile telephone terminal is performed in the following manner.
At the cellular mobile telephone terminal, a target value for the transmit power necessary to keep the received signal strength at the base station at a constant value is set based on the received signal strength at the cellular mobile telephone terminal, and control is performed so that the transmit power matches the target value by forming a feedback control loop in which the actual transmit power is constantly compared with the target value to cause the transmit power to follow the target value.
Next, the configuration and operation of a prior art cellular mobile telephone terminal will be described with reference to FIG. 15. As shown in FIG. 15, the cellular mobile telephone terminal is constructed with microcomputer logic blocks, etc. and comprises a baseband section 100 which processes voice signals and a radio transceiver section 200 which takes as an input the voice signal processed by the baseband section 100 and performs communications with a base station.
The radio transceiver section 200 comprises a transmitter section 210 which generates signals for transmission to the base station and a receiver section 220 which receives signals transmitted from the base station.
The transmitter section 210 comprises an intermediate frequency section 230 which performs heterodyning for modulation and frequency conversion of the voice signal supplied from the baseband section 100, and a radio frequency section 240 which amplifies the radio frequency signal output from the intermediate frequency section 230 and supplies it to an antenna 300 via a duplexer 310.
The intermediate frequency section 230 comprises a modulator 231, a variable gain intermediate frequency amplifier 232 for amplifying the output signal of the modulator 231 with a variable gain, and a mixer 233 for converting the output of the variable gain intermediate frequency amplifier 232 into a radio frequency signal. The variable gain intermediate frequency amplifier 232 is usually constructed using a bipolar transistor. The variable gain intermediate frequency amplifier 232 is capable of varying its gain with a linearity of xc2x11 dB over a range of about 40 dB. In this case, the gain is controlled in a continuous manner over a range of about 40 dB using a continuously varying gain control voltage.
The radio frequency section 240 comprises a variable gain radio frequency amplifier 241 for amplifying the radio frequency signal output from the intermediate frequency section 230 and a power amplifier 242 for amplifying the power of the output signal of the variable gain radio frequency amplifier 241. The variable gain radio frequency amplifier 241 is capable of varying its gain with a linearity of xc2x13 dB over a range of about 30 dB. In this case, the gain is controlled in several steps, for example, in three steps, using a gain control voltage that takes discrete values.
The variable gain radio frequency amplifier 241 comprises a front-end amplifier (medium power amplifier) 243 and an attenuator 244 for varying the gain of the radio frequency signal to be input to the power amplifier (high power amplifier) 242 cascaded with the front-end amplifier 243. The attenuator 244 has the function of varying the amount of attenuation with a linearity of xc2x13 dB over a range of about 30 dB. Its gain, however, varies in step-like manner since, as described above, a discrete gain control voltage is given.
The baseband section 100 includes a control section 110. The control section 110 detects the output level of the power amplifier 242 while also detecting the signal strength of the signal received by the receiver section 220, sets an output level target value for the power amplifier 242 based on the signal strength of the received signal, compares the output level of the power amplifier 242 with the output level target value of the power amplifier 242, and applies a gain control voltage Vca responsive to the result of the comparison to the attenuator 244 and a gain control voltage Vcb responsive to the result of the comparison to the variable gain intermediate frequency amplifier 232, thereby controlling the gain of the attenuator 244 and the gain of the variable gain intermediate frequency amplifier 232 in a feedback loop so that the output level of the power amplifier 242 matches the output level target value of the power amplifier 242. In this case, as earlier described, the gain of the attenuator 244 is controlled in step-like fashion, while the gain of the variable gain intermediate frequency amplifier 232 is controlled in continuous fashion.
In the above-described cellular mobile telephone terminal, gain control with a linearity of xc2x11 dB over a range of 70 dB or greater is accomplished by combining the gain control of the variable gain intermediate frequency amplifier 232 with the gain control of the variable gain radio frequency amplifier 241. According to the IS-95 standard, the input stage of the mixer 233 operates in the 200 MHz band, and the output stage of the mixer 233 operates in the 837 MHz band. The signal levels at the various sections when the cellular mobile telephone terminal is operating at a maximum output are +30 dBm at the output end of the power amplifier 242 (0 dBm=1 mW), +5 dBm at the output end of the variable gain radio frequency amplifier 241, xe2x88x9220 dBm at the output end of the mixer 233, and xe2x88x9225 dBm at the output end of the variable gain intermediate frequency amplifier 232.
Assuming here that gain control over a range of 30 dB is performed in the variable gain radio frequency amplifier 241 and gain control over a range of 40 dB in the variable gain intermediate frequency amplifier 232, the signal level at the output end of the variable gain intermediate frequency amplifier 232 varies over a range of xe2x88x9225 dBm to xe2x88x9265 dBm. Further, the signal level at the output end of the mixer 233 varies over a range of xe2x88x9220 dBm to xe2x88x9260 dBm. The signal level at the output end of the variable gain radio frequency amplifier 241 varies over a range of +5 dBm to xe2x88x9265 dBm. The signal level at the output end of the power amplifier 242 varies over a range of +30 dBm to xe2x88x9240 dBm.
Next, the detailed configuration and operation of the attenuator 244 will be described with reference to FIGS. 16 to 18.
FIG. 16 is a circuit diagram showing the configuration of the attenuator 244. The step-like control of the gain is performed using the attenuator 244 as shown here. As shown in FIG. 16, the attenuator 244 comprises a field effect transistor 1 acting as a parallel variable resistor at the input side, capacitors 2, 3, 10, and 11, resistors 5, 7, and 13, a field effect transistor 6 acting as a series variable resistor, and a field effect transistor 9 acting as a parallel (shunt) variable resistor at the output side. The attenuator 244 is provided with a gain control voltage application terminal 4 at which the gain control voltage Vca is applied, a source voltage application terminal 8 at which the supply voltage VDD is applied, a gate voltage application terminal 12 at which GND potential (reference potential) is applied, an input terminal 14 as a radio frequency signal input part, and an output terminal 15 as a radio frequency signal output part. The input terminal 14 is connected to the output end of the mixer 233 in FIG. 15, while the output terminal 15 is connected to the input end of the front-end amplifier 243. Here, the capacitors 2, 3, 10, and 11 act to block the application of DC voltages, and the resistors 5, 7, and 13 each act to prevent the infiltration of radio frequency signals.
FIG. 17 is a diagram showing the gain control characteristics of the attenuator.
The operation of the attenuator having the above-described configuration will be explained. The cellular mobile telephone terminal here operates with a maximum voltage of about 3.0 V supplied from a lithium battery or the like. The threshold voltage of each field effect transistor refers to the bias required to cause the variable resistor to start its gain control operation. The field effect transistors 6, 1, and 9 forming the series variable resistor and the parallel variable resistors, respectively, are chosen to have the same threshold voltage. Designated voltages are applied at the source voltage application terminal 8 of the field effect transistor 6 and the gate voltage application terminal 12 of the field effect transistors 1 and 9.
When a voltage of 0 to 1.1 V is applied as the gain control voltage Vca to the gain control voltage application terminal 4 (the gain control voltage range (a) in FIG. 17), the resistance value RON (T-FET) of the field effect transistor 6 is at its maximum level, while the resistance value RON (S-FET) of the field effect transistors 1 and 9 is held at its minimum level; as a result, the signal input via the input terminal 14 is attenuated with no increase in gain, and the output signal POUT from the output terminal 15 is at its minimum level.
When the voltage applied at the gain control voltage application terminal 4 is increased above 1.1 V (the gain control voltage range (b) in FIG. 17), the resistance value RON (T-FET) of the field effect transistor 6 begins to decrease, while the resistance value RON (S-FET) of the field effect transistors 1 and 9 remains held at its minimum level; as a result, the output signal POUT increases. Usually, the gain control voltage operating range of a variable resistor implemented by a field effect transistor is about 0.2 to 0.3 V wide; therefore, the gain increases linearly by 18 dB until the voltage applied at the gain control voltage application terminal 4 reaches 1.4 V.
When the voltage applied at the gain control voltage application terminal 4 reaches 1.4 V (the gain control voltage range (c) in FIG. 17), the resistance value RON (T-FET) of the field effect transistor 6, which has been decreasing, now reaches its minimum level, while the resistance value RON (S-FET) of the field effect transistors 1 and 9 held at its minimum level begins to increase, so that the output signal POUT further increases. Here, the gain increases linearly by 12 dB, this time with a sensitivity different from that in the gain control voltage range (b) of 1.1 to 1.4 V, until the voltage applied at the gain control voltage application terminal 4 reaches 1.7 V.
When the voltage applied at the gain control voltage application terminal 4 reaches 1.7 V (the gain control voltage range (d) in FIG. 17), the resistance value RON (S-FET) of the field effect transistors 1 and 9 is now at its maximum level, while the resistance value RON (T-FET) of the field effect transistor 6 remains held at its minimum level; as a result, the output signal POUT reaches maximum. At this point, the gain control width of this amplifier is 30 dB. If a voltage greater than 1.7 V is applied to the gain control voltage application terminal 4, the resistance value RON (T-FET) of the field effect transistor 6 remains at its minimum level while the resistance value RON (S-FET) of the field effect transistors 1 and 9 remains at its maximum level, so that the output signal POUT remains unchanged at its maximum level.
In the attenuator having the above-described gain control characteristics, the output level is changed in three steps by selectively applying, for example, three values VCL, VCM, and VCH as the gain control voltage Vca, as shown in FIG. 18.
On the other hand, in the variable gain intermediate frequency amplifier 232, the output level is varied continuously by varying the gain control voltage Vcb, as shown in FIG. 19.
However, if gain control over a range wider than 70 dB is performed by combining the step control in the radio frequency section with the continuous control in the intermediate frequency section, as described above, since the gain control voltage for the continuous control and the control voltage for the step control are simultaneously changed when the control mode is switched from one mode to the next in the step control, gain differences may occur before and after the mode switching because of variations in the characteristics of the attenuator 244 and variable gain intermediate frequency amplifier 232. Here, consider a situation where the cellular mobile telephone terminal is performing communications under ideal conditions while moving away from the base station at a constant speed; in this situation, if the above condition occurs, the output POUT of the cellular mobile telephone terminal, which should normally increase linearly by virtue of its gain control function, momentarily deviates from the linear control line at the instant of mode switching in the step control, as shown in FIG. 20, because of a delay in tracking operation due to a time delay of the feedback control and because of the output level discontinuity at the time of mode switching. If this happens, the received signal strength at the base station deviates from the specified value, causing level differences with respect to the adjacent channels and thus disrupting voice signals and degrading voice communication quality. Though the problem here is described dealing with the case where the cellular mobile telephone terminal is moving under ideal conditions, it will be noted that the actual condition when the cellular mobile telephone terminal is moving around is usually much worse, for example, the cellular mobile telephone terminal might move behind a building, causing an abrupt drop in the received signal strength; in actual situations, therefore, the problem of the received signal strength at the base station deviating from the specified value is expected to occur frequently, resulting in further degradation of the voice communication quality.
Furthermore, since the two kinds of gain control voltages Vca and Vcb for controlling the variable gain radio frequency amplifier 241 and the variable gain intermediate frequency amplifier 231 have to be set in the control section 110 of the baseband section 100, control logic of the control section 110 becomes complex.
Moreover, the variable gain intermediate frequency amplifier 231 has to be provided in the intermediate frequency section 230 in addition to the variable gain radio frequency amplifier 241 provided in the radio frequency section 240; this has lead to the problem that the circuit configuration increases not only in complexity but also in size, increasing the overall size of the cellular mobile telephone terminal.
It is, accordingly, an object of the present invention to provide a cellular mobile telephone terminal capable of achieving high quality voice communications.
It is another object of the present invention to provide a cellular mobile telephone terminal that can simplify the gain control.
It is a further object of the present invention to provide a cellular mobile telephone terminal that can achieve a space saving and compact construction.
A cellular mobile telephone terminal according to the present invention comprises a baseband section for processing a voice signal and a radio transceiver section for taking as an input the voice signal processed by the baseband section and for performing communications with a base station. The radio transceiver section comprises a transmitter section for generating a signal for transmission to the base station and a receiver section for receiving a signal transmitted from the base station. The transmitter section comprises an intermediate frequency section for performing heterodyning for modulation and frequency conversion of the voice signal supplied from the baseband section and a radio frequency section for amplifying a radio frequency signal output from the intermediate frequency section and for supplying the amplified signal to an antenna. The radio frequency section comprises a gain controller for controlling the gain of the radio frequency signal output from the intermediate frequency section and a power amplifier for amplifying the power of an output of the gain controller.
The baseband section includes a control section, and the control section detects the output level of the power amplifier while also detecting signal strength of the signal received by the receiver section, sets an output level target value for the power amplifier based on the signal strength of the received signal, compares the output level of the power amplifier with the output level target value of the power amplifier, and applies to the gain controller a gain control voltage responsive to the result of the comparison, thereby controlling the gain of the gain controller in a feedback loop so that the output level of the power amplifier matches the output level target value of the power amplifier.
The gain controller comprises: at least two series variable resistors formed, for example, field effect transistors, inserted in a signal line connecting between a signal input part and signal output part for the radio frequency signal; and parallel variable resistors formed, for example, from field effect transistors, connected between a ground line and the signal input part and signal output part, respectively, and the gain controller controls the output of the power amplifier linearly and in substantially continuous fashion by controlling the gain of each variable resistor through the gain control voltage which is applied to a gain control voltage application part.
According to the above configuration, the gain controller acting an attenuator is constructed using at least two series variable resistors formed from field effect transistors connected in multiple stages in conjunction with parallel variable resistors, and the gain controller controls the output of the power amplifier linearly and in substantially continuous fashion by controlling the gain of each variable resistor; this eliminates the problem associated with the step-like gain switching, and ensures high quality voice communications. Further, since the gain control need only be performed in the radio frequency section, the gain control can be simplified. Furthermore, since the variable gain intermediate frequency amplifier in the intermediate frequency section can be omitted, space saving and compact construction can be realized.
A cellular mobile telephone terminal according to the present invention comprises a baseband section for processing a voice signal and a radio transceiver section for taking as an input the voice signal processed by the baseband section and for performing communications with a base station. The radio transceiver section comprises a transmitter section for generating a signal for transmission to the base station and a receiver section for receiving a signal transmitted from the base station. The transmitter section comprises an intermediate frequency section for performing heterodyning for modulation and frequency conversion of the voice signal supplied from the baseband section and a radio frequency section for amplifying a radio frequency signal output from the intermediate frequency section and for supplying the amplified signal to an antenna. The radio frequency section comprises a gain controller for controlling the gain of the radio frequency signal output from the intermediate frequency section and a power amplifier for amplifying the power of an output of the gain controller.
The baseband section includes a control section, and the control section detects the output level of the power amplifier while also detecting signal strength of the signal received by the receiver section, sets an output level target value for the power amplifier based on the signal strength of the received signal, compares the output level of the power amplifier with the output level target value of the power amplifier, and applies to the gain controller a gain control voltage responsive to the result of the comparison, thereby controlling the gain of the gain controller in a feedback loop so that the output level of the power amplifier matches the output level target value of the power amplifier.
The gain controller comprises: at least two series variable resistors formed, for example, field effect transistors, inserted in a signal line connecting between a signal input part and signal output part for the radio frequency signal; and parallel variable resistors formed, for example, from field effect transistors, connected between a ground line and the signal input part and signal output part, respectively.
According to the above configuration, the gain controller acting an attenuator is constructed using at least two series variable resistors formed from field effect transistors connected in multiple stages in conjunction with parallel variable resistors, and the operation points of the at least two series variable resistors, formed from field effect transistors connected in multiple stages, are displaced from each other by an amount corresponding to the width of a linear gain control operation range and the linear operation ranges of the respective series variable resistors are concatenated, the combined linear operation range of the series variable resistors then being made substantially continuous with the linear operation range of the parallel variable resistors. In this arrangement, since switching between each mode is made using a single gain control voltage, gain differences do not occur and a highly accurate linear gain control operation can be performed over a wide range of 70 dB or greater.
As a result, the cellular mobile telephone terminal using the gain control described above solves the problem associated with the step-like gain switching and achieves high quality voice communications. Further, since the gain control need only be performed in the radio frequency section, the gain control can be simplified. Furthermore, since the variable gain intermediate frequency amplifier in the intermediate frequency section can be omitted, space saving and compact construction can be realized.
In one configuration where the operation points of the at least two series variable resistors, formed from field effect transistors connected in multiple stages, are displaced from each other by an amount corresponding to the width of the linear gain control operation range, different reference voltages are applied to the sources of the at least two field effect transistors connected in series. With this configuration, extremely accurate gain control can be achieved, as the gain control is performed using a single gain control voltage. The setting of the gain control operation voltage can be changed as desired.
In another configuration, different gain control voltages are applied to the gates of the at least two field effect transistors connected in series. With this configuration, since the same reference voltage is applied to the sources of the at least two field effect transistors acting as the series variable resistors, accurate and linear gain control can be achieved despite fluctuations in the reference voltage. The setting of the gain control operation voltage can be changed as desired. In the above-described configuration, different gain control voltages are applied to the gates of the two or more field effect transistors, but they can in effect be regarded as one control voltage since the voltage value is simply shifted.
In still another configuration, field effect transistors having different threshold voltages are employed for the at least two field effect transistors connected in series. With this configuration, extremely accurate gain control can be achieved, as the gain control is performed using a single gain control voltage. Furthermore, since the same reference voltage is applied to the sources of the at least two field effect transistors acting as the series variable resistors, accurate and linear gain control can be achieved despite fluctuations in the reference voltage. Moreover, this configuration serves to simplify the circuit configuration, because the number of voltages to be applied can be reduced.